Add flags for switching off speculative evaluation.

We found that speculative evaluation can increase the amount of
allocations in some circumstances. This patch adds new flags for
selectively disabling speculative evaluation, allowing us to
test the effect of the optimization.

The new flags are:

  -fspec-eval
     globally enable speculative evaluation

  -fspec-eval-dictfun
     enable speculative evaluation for dictionary functions (no effect
     if speculative evaluation is globally disabled)

The new flags are on by default for all optimisation levels.

See #25284